Virology: IgG avidity in congenital infection: practice question

A Microbiology question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Microbiology · Virology: IgG avidity in congenital infection

Sample item

A woman at 14 weeks of gestation has positive CMV IgM and IgG on screening. Which additional test best helps decide whether the infection was acquired recently?

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

C. Measurement of CMV IgG avidity

IgM can persist for months and may be non-specific, so it cannot date infection. Low-avidity IgG indicates a primary infection within roughly the previous three months, whereas high avidity makes a recent primary infection unlikely.

Option by option

  • A. Culture of a throat swab for CMV

    Throat culture only shows shedding, which also occurs in old or reactivated infection.

  • B. Repeat IgM titre after two weeks

    Repeating IgM does not date the infection because IgM may stay positive for months.

  • C. Measurement of CMV IgG avidity (correct)

    Correct. Low-avidity IgG indicates recent primary infection, and high avidity excludes it.

  • D. Complement fixation on a single sample

    A single complement fixation titre cannot distinguish recent from past infection.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

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Questions about this Microbiology question

What is the answer to this Microbiology question?

The best answer is C, Measurement of CMV IgG avidity. IgM can persist for months and may be non-specific, so it cannot date infection.

Why is "Culture of a throat swab for CMV" not the answer?

Throat culture only shows shedding, which also occurs in old or reactivated infection.

Why is "Repeat IgM titre after two weeks" not the answer?

Repeating IgM does not date the infection because IgM may stay positive for months.

Why is "Complement fixation on a single sample" not the answer?

A single complement fixation titre cannot distinguish recent from past infection.

What concept does this question test?

It tests Virology: IgG avidity in congenital infection in Microbiology. Low-avidity IgG indicates a primary infection within roughly the previous three months, whereas high avidity makes a recent primary infection unlikely.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Virology: IgG avidity in congenital infection?

Make a one-line identity card for each organism: shape and stain, defining test, toxin, disease and drug. Then use the flashcards and the other Microbiology questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.